I'd try the usual stuff first like using the eject disc in other applications (such as Disk Utility or Toast) and/or hold the eject button on the keyboard during startup. I've experienced that some macs take their time to eject the disc so in a few cases it's not enough to just press the eject button on the keyboard, you have to hold it down for a few seconds... Other than that you can always try to go through OFW (hold command, option, O and F during startup) and type "eject cd".